// JavaScript Document
    var canvas = document.getElementById("cas");

    var ctx = canvas.getContext("2d");
    resize();

    window.onresize = resize;




    function resize() {

    canvas.width = window.innerWidth || document.documentElement.clientWidth || document.body.clientWidth;

    canvas.height = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight;

    }




    var RAF = (function() {

    return window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || window.oRequestAnimationFrame || window.msRequestAnimationFrame || function(callback) {

    window.setTimeout(callback, 1000 / 60);

    };

    })();




    // 鼠标活动时，获取鼠标坐标

    var warea = {x: null, y: null, max: 20000};

    window.onmousemove = function(e) {

    e = e || window.event;




    warea.x = e.clientX;

    warea.y = e.clientY;

    };

    window.onmouseout = function(e) {

    warea.x = null;

    warea.y = null;

    };




    // 添加粒子

    // x，y为粒子坐标，xa, ya为粒子xy轴加速度，max为连线的最大距离

    var dots = [];

    for (var i = 0; i < 300; i++) {

    var x = Math.random() * canvas.width;

    var y = Math.random() * canvas.height;

    var xa = Math.random() * 2 - 1;

    var ya = Math.random() * 2 - 1;




    dots.push({

    x: x,

    y: y,

    xa: xa,

    ya: ya,

    max: 6000

    })

    }




    // 延迟100秒开始执行动画，如果立即执行有时位置计算会出错

    setTimeout(function() {

    animate();

    }, 100);




    // 每一帧循环的逻辑

    function animate() {

    ctx.clearRect(0, 0, canvas.width, canvas.height);




    // 将鼠标坐标添加进去，产生一个用于比对距离的点数组

    var ndots = [warea].concat(dots);




    dots.forEach(function(dot) {




    // 粒子位移

    dot.x += dot.xa;

    dot.y += dot.ya;




    // 遇到边界将加速度反向

    dot.xa *= (dot.x > canvas.width || dot.x < 0) ? -1 : 1;

    dot.ya *= (dot.y > canvas.height || dot.y < 0) ? -1 : 1;




    // 绘制点

    ctx.fillRect(dot.x - 0.5, dot.y - 0.5, 1, 1);




    // 循环比对粒子间的距离

    for (var i = 0; i < ndots.length; i++) {

    var d2 = ndots[i];




    if (dot === d2 || d2.x === null || d2.y === null) continue;




    var xc = dot.x - d2.x;

    var yc = dot.y - d2.y;




    // 两个粒子之间的距离

    var dis = xc * xc + yc * yc;




    // 距离比

    var ratio;




    // 如果两个粒子之间的距离小于粒子对象的max值，则在两个粒子间画线

    if (dis < d2.max) {




    // 如果是鼠标，则让粒子向鼠标的位置移动

    if (d2 === warea && dis > (d2.max / 2)) {

    dot.x -= xc * 0.03;

    dot.y -= yc * 0.03;

    }




    // 计算距离比

    ratio = (d2.max - dis) / d2.max;




    // 画线

    ctx.beginPath();

    ctx.lineWidth = ratio / 2;

    ctx.strokeStyle = 'rgba(0,0,0,' + (ratio + 0.2) + ')';

    ctx.moveTo(dot.x, dot.y);

    ctx.lineTo(d2.x, d2.y);

    ctx.stroke();
    }
    }

    // 将已经计算过的粒子从数组中删除
    ndots.splice(ndots.indexOf(dot), 1);
    });
    RAF(animate);
    }